Zinc is an element that is an essential trace element of soil and is often lost from soils by leaching during weathering and soil formation so that many soils contain less than 50 ppm zinc (Gilkes CSBP and Farmers Ltd.--Productivity Focus, January 1994). In this reference, it is also stated that zinc fertilizers produce substantial increases in the yield and quality of grains, pastures, fruits, fibres and animals and thus it may be concluded that zinc uptake in soil and plants is an extremely important criterion of zinc fertilizer efficiency in use.
Zinc fertilizers are discussed in Chapter 3 of the Proceedings of the International Symposium on "Zinc and Soil and Plants" which was held at The University of Western Australia on Sep. 27-28, 1993 and which has now been published by Kluwer Academic Publishers and edited by A. D. Robson. In this reference, it is stated that the four most common classes of zinc fertilizers include:
(i) inorganic sources such as ZnSO.sub.4, ZnO, ZnCO.sub.3, Zn(NO.sub.3).sub.2 and ZnCl.sub.2 ; PA1 (ii) synthetic chelates inclusive of Zn (EDTA); PA1 (iii) natural organic complexes; and PA1 (iv) inorganic complexes such as Zn(NH.sub.3).sub.4 and ZnSO.sub.4. PA1 (i) mixing solid pre-manufactured ammonium phosphate having an N/P ratio of 0.6-1.9 and a zinc source in the form of a slurry, suspension or particles with each other to form a resulting co-granulate in a granulator vessel; and PA1 (ii) drying the co-granulate.
ZnSO.sub.4 is the most common source and is sold in both crystalline and granular form. This is also true of ZnO but the effectiveness of ZnO is low when sold in granular form because of its very low solubility in water.
The most common method of application of zinc is soil application. Zinc fertilizers are mainly applied to soils in combination with NP (nitrogen-phosphorus) fertilizers which may include di-ammonium phosphate (DAP) or mono-ammonium phosphate (MAP) either by incorporation by granulation or bulk blending in granular form with other granular fertilizers. Granular NP fertilizers are used as carriers for Zn because it allows for more uniform distribution with conventional granulation apparatus which is described, for example, in International Publication No. WO95/21689.
However, when zinc sources are incorporated in NP fertilizers during wet granulation, under the relevant conditions of high temperature and moisture, chemical reactions may reduce the plant availability of some zinc sources. For example, insoluble zinc ammonium phosphate (ZnNH.sub.4 PO.sub.4) may be formed in the presence of ammonium ion and phosphate ion in the granulator. Thus zinc ammonium phosphate is formed which is not available for crops, especially in sandy neutral or alkaline soils under dry conditions. In another example, when a synthetic chelate such as ZnEDTA is mixed with phosphoric acid before ammoniation, acid decomposition of the chelate molecule results in decreased availability of some Zn fertilizers as described in Mortvedt, 1968, J. Agr. Food Chem. 16 241-245.
In relation to bulk blending Zn fertilizers with granular NP fertilizers, one advantage that is obtained by this procedure is that fertilizer grades can be produced which will require the recommended rates of Zn, N and P for a given targeted yield. However, the main disadvantage of this procedure is that segregation of Zn can occur during the blending operation and with subsequent handling. Segregation results in non-uniform application which is critical with Zn because the application rate of Zn is low. Segregation mainly occurs because of the fact that the particle size of Zn fertilizer is substantially less than that of the NP granules and thus the blended fertilizer will have localized pockets of excessively high concentrations of Zn.
Coating Zn fertilizers onto granular NP fertilizers may eliminate the possibility of segregation. The Zn fertilizer should be ground to a finely divided state such as less than 0.25 mm to adhere to the NP granules. However, the coating method has been found in some cases to be ineffective because of separation between the NP fertilizer core and the Zn coating whereby the coating falls off and provides a substantial "dust" or finely divided particulate material which renders the resulting fertilizer less ineffective in use as well as being of nuisance value.
Reference is made to U.S. Pat. No. 4,154,593 which relates to a process of granulating ammonium phosphate wherein the ammonium phosphate is subjected to a shearing operation in a kneading mill or pug mill and is also ammoniated prior to being passed into a rotary drum granulator in the form of a slurry or melt. Additional ammonium sulphate may also be passed into the granulator in the form of a slurry or melt as well as additional amounts of ammonia. A heat of reaction is then generated in the granulator wherein at least a portion of the reactable components of the slurry or melt are reacted. This reference also refers to the addition of zinc as a solid filler also being introduced into the granulator.
However, it is considered that as ammonia and ammonium phosphate in the form of a slurry or melt are added to the granulator, then this will mean that the zinc will react with the ammonium phosphate or ammonia to form a resulting complex so as to inhibit the zinc from being freely available for uptake into soil and plants. In other words, the available zinc is "locked up" and uptake is prevented.
AU 554749 refers to a process for producing micro-nutrient containing fertilizer wherein particulate phosphate containing fertilizer, such as ammonium phosphate, is treated with a mineral acid and the fertilizer material is tumbled in the presence of particulate micro-nutrient material inclusive of a zinc compound so that the zinc compound is bonded to external surfaces of the fertilizer material while the external surfaces are wetted with acid. This reference, therefore, is an example of coating granular NP fertilizers with micro-nutrient and this therefore has the disadvantage of separation occurring as described above.
U.S. Pat. No. 3,560,192 describes a similar coating process wherein the fertilizer granules are coated with micro-nutrient in the presence of a binder which is a solution of zinc chloride. However, the micro-nutrient is still applied to external surfaces of the fertilizer granules and separation may still occur as described above in relation to AU 554749.
RU 2034817 refers to production of granulated fertilizer which comprises adding a mixture of calcium carbonate and magnesium carbonate to ammonium phosphates and wherein the resulting mixture is granulated at 80-100.degree. C. The problem of zinc uptake in soil and plants is not discussed in this reference.
Similar comments apply to GB 1263719 which refers to tumbling or agitating ammonium phosphate, urea and a potassium salt at an elevated temperature in the presence of 0.5-5.0% by weight of water based on the mixture and drying the resultant granules at less than 90.degree. C. The addition of zinc is not specifically mentioned in this reference and the problem of enhancing zinc uptake in plants or soil is not addressed.
BE 861277 refers to ammonium sulphate fertilizers which also contain the trace elements, copper and zinc, wherein 0.2-0.6 parts of copper and 0.7-2.0 parts of zinc are incorporated per 1000 parts of total composition. These compositions are prepared by dissolving the trace elements in phosphoric acid and treating the resulting solution with ammonia until the N/P ratio is about 2.0. In this reference, the problem of enhancing zinc uptake in plants and soil is not addressed and, in any event, the zinc would be presumably "locked up" because of reaction with diammonium phosphate during granulation or by reaction with phosphoric acid.
SU 1481230 refers to addition of theophylline, a zinc containing additive to wet process phosphoric acid, neutralization with ammonia, drying the slurry so obtained and granulating the slurry. This reference does not teach the reaction of ammonium phosphate with zinc and, in any event, the zinc would not be available for uptake because of its reaction with phosphoric acid.
AU 445640 refers to preparation of granular fertilizers wherein initially particulate ammonium phosphate having an initial N:P atomic ratio in the range of 0.8:1 to 0.95:1 are granulated in the presence of water at a temperature of 30-100.degree. C. and treating such particulate material with ammonia during granulation. The particulate material may also contain potassium chloride, ammonium nitrate and urea. The use of zinc is not specifically mentioned in this reference.
Reference may also be made to the Indian Journal of Agronomy, 1989, 34(4) 487-488 which refers to studying the effects of zinc coated fertilizers wherein the fertilizer is ammonium phosphate in relation to rice. This reference suffers from the same problems referred to above wherein the zinc coating may separate from the fertilizer.
Reference may also be made to Koshino, Soil Science and Plant Nutrition, 1974, 20 331 or the Journal of the Science of Soil and Manure, Tokyo, 1973, 44(6) 217-222 wherein reference is made to ammonium phosphate coated with zinc and zinc uptake in soils. Similar comments can therefore be made in relation to this reference as made above in relation to AU 554749 or U.S. Pat. No. 3,560,192.
In Giordano et al., 1978, Agronomy Journal 70 531-534, reference is made to pot trials where fertilizer pellets are made by pressing the powdered fertilizer and micro-nutrient into pellets in a press. This reference, therefore, makes no reference to a commercial granulation technique.
Reference may also be made to Daug et al., 1992 Indian Journal of Agricultural Research, 26(2) 91-99, which refers to zincated fertilizers prepared by dry and wet blending including use of diammonium phosphate (DAP) as the fertilizer. Again, this reference does not refer to use of commercial granulation techniques and it would seem that in relation to the blend of DAP and zinc compound that no improvement was noted in relation to zinc uptake in soil or plants.